Self-repairing cements

ABSTRACT

A self-adaptive cement formulation includes cement, water and thermoplastic block-polymer particles. The set cement demonstrates self-healing properties when exposed to methane, and is particularly suited for well-cementing applications. After placement and curing, the self healing properties help maintain zonal isolation should bonding be disrupted between the set cement and the formation or a casing string, should cracks or defects appear in the set-cement matrix, or both.

TECHNICAL FIELD

The present disclosure relates to self-adaptive cements. In particular,it relates to set cements that are “self-healing,” i.e., formulationsthat can adapt to compensate for changes or faults in the physicalstructure of the cement, or which adapt their structure after thesetting phase of the cement in the cementing of oil, gas, water orgeothermal wells, or the like.

BACKGROUND

During the construction of wells, cement is used to secure and supportcasing inside the well and prevent fluid communication between thevarious underground fluid-containing layers or the production ofunwanted fluids into the well.

Various approaches have been developed to prevent failure of the cementsheath. One approach is to design the cement sheath to take into accountphysical stresses that might be encountered during its lifetime. Such anapproach is described for example in U.S. Pat. No. 6,296,057. Anotherapproach is to include, in the cement composition, materials thatimprove the physical properties of the set cement. U.S. Pat. No.6,458,198 describes the addition of amorphous metal fibers to the cementslurry to improve its strength and resistance to impact damage. EP1129047 and WO 00/37387 describe the addition of flexible materials(rubber or polymers) to the cement to confer a degree of flexibility onthe cement sheath.

Nevertheless, the above-described approaches do not allow restoration ofthe zonal isolation once the cement sheath has actually failed due tothe formation of cracks or microannuli.

A number of self-healing concretes are known for use in the constructionindustry. These are described for example in U.S. Pat. Nos. 5,575,841,5,660,624, 5,989,334, 6,261,360 and 6,527,849, and in the documententitled “Three designs for the internal release of sealants, adhesives,and waterproofing chemicals into concrete to reduce permeability”, Dry,C. M., Cement and Concrete Research 30 (2000) 1969-1977.

Nevertheless, none of these self-healing concretes are immediatelyapplicable to well cementing operations because of the need for thematerial to be pumpable during placement.

“Self-healing” cements were eventually developed for oil and gasindustry applications such as described in US 2007/0204765 A1, WO2004/101951 and WO 2004/101952 A1. These formulations generally containadditives that react and/or swell upon contact with downhole fluids.When cement-sheath deterioration occurs, exposing the cement matrix orcement-sheath surfaces to downhole fluids, the additives respond andseal cracks or fissures, thereby restoring cement-matrix integrity andzonal isolation. Well cements are potentially exposed to several fluidtypes during service, including liquid and gaseous hydrocarbons, water,brines and/or carbon dioxide. Thus, depending on the anticipatedwellbore environment, it would be desirable to incorporate additivesthat are able to respond to one or more types of downhole fluids.

Despite the many valuable contributions from the art, it would bedesirable to have access to a self-healing set cement that responds toformation fluids that contain high concentrations of gaseoushydrocarbons.

SUMMARY

The present disclosure provides set cements that are self-healing whenexposed to hydrocarbons, and methods by which they may be prepared andapplied in subterranean wells.

In an aspect, embodiments relate to methods for maintaining zonalisolation in a subterranean well that penetrates one or morehydrocarbon-containing formations.

In a further aspect, embodiments relate to uses of thermoplasticblock-polymer particles to impart self-healing properties to a cementformulation that is placed in a subterranean well penetrating one ormore hydrocarbon-containing formations.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot showing the swelling characteristics ofstyrene-isoprene-styrene (SIS) and styrene-butadiene-styrene (SBS)particles in the presence of methane at various temperatures andpressures.

FIG. 2 is a schematic diagram of an experimental apparatus for measuringthe self-healing ability of fractured cement samples.

FIG. 3 presents normalized flow-rate reductions for set cementscontaining SIS and SBS particles exposed to methane.

FIG. 4 presents the effect of slurry density on normalized flow-ratereductions for set cements containing SIS and SBS particles exposed tomethane.

FIG. 5 presents normalized flow-rate reductions for set cementscontaining SIS and SBS particles exposed to methane at variouspressures.

DETAILED DESCRIPTION

At the outset, it should be noted that in the development of any suchactual embodiment, numerous implementations—specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem related and business related constraints, which will vary fromone implementation to another. Moreover, it will be appreciated thatsuch a development effort might be complex and time consuming but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure. In addition, the compositionused/disclosed herein can also comprise some components other than thosecited. In the summary and this detailed description, each numericalvalue should be read once as modified by the term “about” (unlessalready expressly so modified), and then read again as not so modifiedunless otherwise indicated in context. Also, in the summary and thisdetailed description, it should be understood that a concentration rangelisted or described as being useful, suitable, or the like, is intendedthat any and every concentration within the range, including the endpoints, is to be considered as having been stated. For example, “a rangeof from 1 to 10” is to be read as indicating each and every possiblenumber along the continuum between about 1 and about 10. Thus, even ifspecific data points within the range, or even no data points within therange, are explicitly identified or refer to only a few specific points,it is to be understood that inventors appreciate and understand that anyand all data points within the range are to be considered to have beenspecified, and that inventors possessed knowledge of the entire rangeand all points within the range.

This disclosure concerns compositions for cementing subterranean wells,comprising a settable material, water and at least one Additive thatswells in the event of structural failure of or damage to the setmaterial (i.e., the cement sheath). Such behavior restores and maintainsa physical and hydraulic barrier in the failure zone. As a result, zonalisolation in the subterranean well is preserved. Such set cements aresaid to be “self-healing” or “self-repairing.” In this application, bothterms are used indifferently, and are to be understood as the capacityof a cement sheath to restore hydraulic isolation after suffering amatrix-permeability increase, structural defects such as cracks orfissures, or debonding from casing or formation surfaces (i.e.,microannuli).

Examples of settable materials include (but are not limited to) Portlandcement, microcement, geopolymers, mixtures of cement and geopolymer,plaster, lime-silica mixtures, resins, phosphomagnesium cements orchemically bonded phosphate ceramics (CBPCs).

As stated earlier, there is a need for self-healing set cements thatoperate in an environment containing high concentrations of gaseoushydrocarbons, methane in particular. Surprisingly, the inventors havediscovered that self-healing properties may be achieved in thisenvironment by incorporating thermoplastic block-polymer particles inthe cement formulation. Typical block polymers comprise alternatingsections of one chemical compound separated by sections of a differentchemical compound, or a coupling group of low molecular weight. Forexample, block polymers can have the structure (A-b-B-b-A), wherein Arepresents a block that is glassy or semi-crystalline and B is a blockthat is elastomeric. In principle, A can be any polymer that is normallyregarded as thermoplastic (e.g., polystyrene, polymethylmethacrylate,isotactic polypropylene, polyurethane, etc.), and B can be any polymerthat is normally regarded as elastomeric (e.g., polyisoprene,polybutadiene, polyethers, polyesters, etc.).

Further embodiments relate to methods for maintaining zonal isolation ina subterranean well having a borehole that penetrates one or morehydrocarbon-containing formations. The method comprises pumping a cementslurry comprising thermoplastic block-polymer particles into the well,and allowing the cement slurry to form a cement sheath. Those skilled inthe art will recognize that a cement slurry is generally considered tobe pumpable when its viscosity is less than or equal to 1000 mPa-s at ashear rate of 100 s⁻¹, throughout the temperature range the slurry willexperience during placement in the well. The cement sheath may belocated between the well casing and the borehole wall, or between thecasing and another casing string. If microannuli, cracks or defectsoccur in the cement sheath, the casing-cement interface or thecement-borehole wall interface, the particles will be exposed toformation hydrocarbons, causing them to swell and enabling the cementsheath to have self-healing properties.

Yet further embodiments aim at uses of thermoplastic block-polymerparticles to impart self-healing properties to a set cement sheath in asubterranean well that penetrates one or more hydrocarbon-containingformations. The particles swell when contacted by hydrocarbons from theformation, in particular gaseous hydrocarbons.

For all aspects, the tensile strength of the block polymer may be variedbetween (but is not limited to) about 1.5 MPa and 40 MPa, preferablybetween 3.4 to 34 MPa. Even more preferred tensile-strength may bebetween 2 MPa and 3.45 MPa or between 28 MPa and 34 MPa.

Preferred thermoplastic block polymers include styrene-isoprene-styrene(SIS), styrene-butadiene-styrene (SBS) and mixtures thereof. Theblock-polymer-additive may be in one or more shapes, including (but notlimited to) spherical, ovoid, fibrous, ribbon-like and in the form of amesh.

The concentration of the block-polymer particles is preferably betweenabout 10% and 55% by volume of solids in the cement slurry, also knownas percentage by volume of blend (BVOB). A more preferred particleconcentration lies between about 20% and 50% BVOB. The particle-sizerange is preferably between about 100 μm and 900 μm, and more preferablybetween about 200 μm and 800 μm.

One of the current challenge that the industry is facing is the presencein some wells of high concentration of gaseous hydrocarbons such asmethane, propane and/or ethane. Such gaseous hydrocarbons being muchmore volatile than hydrocarbons in liquid form have the tendency topenetrate the failures and/or microannuli that can be present and thecement sheath and thus modifying the pressure and safety conditions ofthe well as the integrity is diminished. The inventors have determinedthat the present compositions can solve this problem up to very highconcentration of gaseous hydrocarbon. In a preferred embodiment, thegaseous concentration of hydrocarbon fluid is greater than about 91 mol%, and more preferably above about 95 mol %. In addition, thehydrocarbon pressure to which the cement sheath is exposed is preferablyabove about 3.5 MPa, more preferably above about 6.9 MPa and mostpreferably above about 13.7 MPa.

The block-polymer particles may be further encapsulated by a protectivelayer. The layer may rupture or degrade upon exposure to one or moretriggers, including (but not limited to) contact with a hydrocarbon,propagation of a crack within the set-cement matrix, time and/ortemperature.

In addition to the block-polymer particles, the cement slurries may alsocomprise customary additives such as retarders, accelerators, extenders,fluid-loss-control additives, lost-circulation additives, gas-migrationadditives and antifoam agents. Furthermore, the cement slurries maycontain additives that enhance the flexibility and/or toughness of theset cement. Such additives include (but are not limited to) flexibleparticles having a Young's modulus below about 5000 MPa and a Poisson'sratio above about 0.3. Preferably, such particles would have a Young'smodulus below about 2000 MPa. Examples include (but are not limited to)polypropylene, polyethylene, acrylonitrile butadiene, styrene butadieneand polyamide. Such additives may also include fibers selected from thelist comprising polyamide, polyethylene and polyvinyl alcohol. Metallicmicroribbons may also be included.

The block-polymer particles may also be used in engineered-particle-sizecement formulations involving trimodal or quadrimodal blends of small,medium and coarse particles. Such as formulations exemplified in U.S.Pat. No. 5,518,996 and/or CA 2,117,276.

The block-polymer particles may be further associated with one or morecompounds from the list comprising an aqueous inverse emulsion ofpolymer comprising a betaine group, poly-2,2,1-bicyclo heptene(polynorbornene), alkylstyrene, crosslinked substituted vinyl acrylatecopolymers, diatomaceous earth, natural rubber, vulcanized rubber,polyisoprene rubber, vinyl acetate rubber, polychloroprene rubber,acrylonitrile butadiene rubber, hydrogenated acrylonitrile butadienerubber, ethylene propylene diene monomer, ethylene propylene monomerrubber, styrene-butadiene rubber, styrene/propylene/diene monomer,brominated poly(isobutylene-co-4-methylstyrene), butyl rubber,chlorosulphonated polyethylenes, polyacrylate rubber, polyurethane,silicone rubber, brominated butyl rubber, chlorinated butyl rubber,chlorinated polyethylene, epichlorohydrin ethylene oxide copolymer,ethylene acrylate rubber, ethylene propylene diene terpolymer rubber,sulphonated polyethylene, fluoro silicone rubbers, fluoroelastomer andsubstituted styrene acrylate copolymers.

Those skilled in the art will appreciate that the disclosed method anduse may not necessarily be applied throughout the entire length of thesubterranean interval being cemented. In such cases, more than onecement-slurry composition is placed sequentially. The first slurry iscalled the “lead,” and the last slurry is called the “tail.” Under thesecircumstances, it is preferred that the inventive slurry be placed suchthat it resides in regions where hydrocarbons exist. In most cases, thiswill be at or near the bottom of the well; therefore, the inventivemethod and use would preferably apply to the tail. Those skilled in theart will also appreciate that the disclosed method and use would notonly be useful for primary cementing, but also for remedial cementingoperations such as squeeze cementing and plug cementing.

Other and further objects, features and advantages will be readilyapparent to those skilled in the art upon a reading of the descriptionof the examples which follows, taken in conjunction with theaccompanying drawings.

EXAMPLES

The following examples serve to further illustrate the disclosure.

Table 1 lists the styrene-isoprene-styrene (SIS) polymers andstyrene-butadiene-styrene (SBS) polymers that were used in the examples.

TABLE 1 Suppliers and Properties of SIS and SBS Polymers Employed inExamples.* Property SIS #1 SIS #2 SBS #1 SBS #2 SBS #3 SBS #4 SupplierICO Kraton ICO ICO ICO Kraton Polymers Polymers Polymers PolymersProduct Name ICO1 D1161 ICO 3 ICO 4 ICO 5 D1192 EM PTM Melt Index 1313.5 <1 23-37 <1 <1 (200° C./5 kg) (g/10 min) Density (kg/m³) 963 920940 940 981 940 Tensile strength 17 21 16 10 33 at break (MPa) Hardness,24 32 72 70 Shore A (30 s) Elongation at 1400 1300 680 900 880 break (%)*Test methods: ISO 1133 (Melt Index measurement) ISO 37 (TensileStrength at Break and Elongation at Break measurements) ISO 2781(Density measurement) ISO 868 (ICO Polymers) and ASTM 2240 (Kraton)(Hardness measurement)

Example 1

Several polymer particles were placed inside a pressure cell equippedwith a window that allows one to observe the behavior of materialswithin the cell. The cell supplier was Temco Inc., Houston, Tex. (USA).The cell temperature was also adjustable. A camera captured images frominside the pressure cell, and image-analysis software was employed tointerpret the behavior of materials inside the cell. For particle-sizemeasurements, the software examined the cross-sectional area of theparticles in the cell.

After the polymer particles were introduced into the cell, the cell wassealed. The cell was then heated to the desired temperature. The initialparticle sizes were measured.

A methane-gas line was then connected to the cell, and the methanepressure was raised to 21 MPa over a 3-min period. The cell pressure wasmaintained for 2 hr, after which the particle sizes were measured again.

Tests were performed at 22° C. and 42° C. with an SIS polymer (SIS #1from Table 1) and an SBS polymer (SBS #3). The results are presented inFIG. 1. At both temperatures, both SIS and SBS polymer demonstrated goodperformance.

Example 2

The properties of cement slurries containing SIS or SBS particles weremeasured. The tests conformed to standard methods published by theInternational Organization for Standards (ISO): “Petroleum and naturalgas industries—Cements and materials for well cementing—Part 2: Testingof well cements,” International Organization for Standards PublicationNo. ISO 10426-2. Two cement slurries were tested—one containing SISparticles (SIS #1), and the other containing SBS particles (SBS #3). Thetest conditions were as follows—bottomhole static temperature: 53° C.;bottomhole circulating temperature: 44° C.; bottomhole pressure: 21 MPa(3000 psi).

The composition of the slurry containing SBS is given in Table 2, andthe test results are presented in Tables 3 and 4. The slurry density was1606 kg/m³, and the solid volume fraction (SVF) of the slurry was 51.8%.

TABLE 2 Composition of Test Cement Slurry Containing SBS as Self-HealingParticle. Quantity Component Type (kg/m³) Cement Class G Portland cement696 Self-healing particle SBS #3 214.5 Silica 200 mesh (74 μm) 200.5Water Fresh 395 Lightweight particle Acrylonitrile-butadiene copolymer 5Antifoam Polypropylene glycol 4 Dispersant Polymelamine sulfonate 9Antisettling 90% crystalline silica; 10% 1 polysaccharide biopolymerFluid-loss additive RHODOFLAC ™, available from 72 Rhodia NederlandRetarder Calcium Lignosulfonate 2.5

TABLE 3 Rheological Properties of Test Cement Slurry Containing SBS asSelf-Healing Particle. Mixing 20-min Conditioning PV: 233 cP PV: 219 cPTy: 4.3 kPa (9 lbf/100 ft²) Ty: 8.1 kPa (17 lbf/100 ft²)

TABLE 4 Additional Properties of Test Cement Slurry Containing SBS asSelf-Healing Particle. Measurement Results Free fluid 0.8% Fluid loss 13mL Thickening time 8:53 (to 70 Bc) Compressive 500 psi [3.4 MPa] (UCA)after 23:42 strength 1000 psi [7 MPa] (UCA) after 72:58 development 783psi [5.4 MPa] (crush); 512 psi [3.5 MPa] (UCA) after 24:00 1316 psi [9MPa] crush (996 psi [6.9 MPa] UCA) after 72:00 Tensile strength* 1.9 MPa*Cement was cured for 7 days at 53° C. and 20 MPa before measuringtensile strength.

The composition of the slurry containing SIS is given in Table 5, andthe test results are presented in Tables 6 and 7. The slurry density was1606 kg/m³, and the solid volume fraction (SVF) of the slurry was 51.7%.

TABLE 5 Composition of Test Cement Slurry Containing SIS as Self-HealingParticle. Quantity Component Type (kg/m³) Cement Class G Portland cement694 Self-healing additive SIS #1 208 Antifoam Polypropylene Glycol 5Silica 200 mesh (74 μm) 219 Water Fresh 393 Dispersant PolymelamineSulfonate 8 Antisettling Biopolymer 1 Fluid loss RHODOFLAC ™, availablefrom 81 Rhodia Nederland

TABLE 6 Rheological Properties of Test Cement Slurry Containing SIS asSelf-Healing Particle. Mixing 20-min Conditioning PV: 119 cP PV: 107 cPTy: 6.7 KPa (14 lbf/100 ft²) Ty: 9.1 KPa (19 lbf/100 ft²)

TABLE 7 Additional Properties of Test Cement Slurry Containing SIS asSelf-Healing Particle. Measurement Results Free water 0.3% Thickeningtime 4:13 (to 70 Bearden cconsistency) Compressive strength 500 psi [3.4MPa] after 11:52 development (measured 1000 psi [7 MPa] after 32:00 byUCA) 867 psi [6 MPa] after 24:00 1260 psi [8.7 MPa] after 72:00

Example 3

Various cement formulations containing SIS or SBS were evaluated fortheir self-healing properties. The slurry compositions are presented inTable 8. The formulation that contains acrylonitrile-butadiene copolymerrubber (ABCR) was included as a control with no self-healing capability.

TABLE 8 Slurry Compositions for Self-Healing Tests. Particle type UnitABCR SIS #1 SIS #2 SBS #1 SBS #2 SBS #3 SBS # 4 Density (kg/m³) 15711498 1606 1498 1498 1498 1606 SVF (%) 55 50.3 52.3 50 50.6 50 52Particle (kg/m³) 286 240 210 243 239 243 213 Cement 616 560 645 555 563553 641 Silica 219 199 281 197 200 196 279 Water 436 494 459 498 491 497463 Antifoam* 3 4 3 4 4 6 3 Dispersant* 5 0 5 0 0 3 5 Antisettling* 1 11 1 1 1 1 Retarder* 5 0 0 0 0 0 0 *Antifoam Agent: polypropylene glycol;Dispersant: polymelamine sulfonate; Antisettling Agent: 90% crystallinesilica, 10% polysaccharide biopolymer; Retarder: calcium lignosulfonate.

Each cement slurry was prepared according to the method described in ISOPublication 10426-2, and samples were prepared in the manner required toperform a Brazilian tensile-strength test. This test is also describedin ISO Publication 10426-2. The cement-core samples were 66 mm long and22 mm in diameter. The samples were cured at room temperature andatmospheric pressure. The curing times are presented in Table 9. Columnswith two numbers indicate that two tests were performed.

TABLE 9 Curing times. Particle SIS SIS SBS SBS SBS SBS name ABCR #1 #2#1 #2 #3 #4 Curing 40/121 48 104 101 79/77 78/105 100 time (days)

The samples were fractured by the Brazilian method, then transferred toa steel tube and secured by a scaling cement. As shown in FIG. 2, thesteel tube 101 is 180 mm long. There are two 90-mm sections—one with aninternal diameter of 31.5 mm in diameter, the other with an internaldiameter of 29.5 mm. The fractured cement sample 102 is placed insidethe tube and the sealing cement 103 is applied around the sample. Midwayalong the cement sample, owing to the different tube diameters, there isan edge 104 to prevent the cement sample from sliding.

The composition of the sealing cement was a 1.88-kg/m³ Portland cementslurry containing 2.7 mL/kg polynaphthalene sulfonate dispersant, 2.7mL/kg polysiloxane antifoam agent, 178 mL/kg styrene butidiene latex and2.1% by weight of cement calcium chloride accelerator.

Pure methane was then injected through the fractured samples for 24hours at 21 MPa backpressure and at ambient temperature (20°-23° C.).Flow-rate and pressure variations were recorded, and normalized flowrates were calculated. The results are shown in FIG. 3.

The cement matrices incorporating SIS particles demonstrated normalizedflow-rate reductions greater than 98%. The performance of cementmatrices incorporating SBS particles demonstrated flow-rate reductionsbetween 49% and 97%. The control did not show a flow-rate reduction.

Example 4

Using the methods described in Example 3, the effect of slurry densityon the performance of set cements containing SIS#1 or SBS#3 wasinvestigated. The slurry compositions are shown in Table 10.

TABLE 10 Slurry Compositions for Self-Healing Tests Density (kg/m³) 16061606 1498 1498 SVF (%) 52 51.5 50.3 50.7 Particle type SIS#1 SBS#3 SIS#1SBS#3 Particle (kg/m³) 213 216 240 242.5 Class G cement 641.5 635.5 560554.3 Silica 280 277 199 196 Water 462.5 467.5 494 496.5 Antifoam 5 5 44 Dispersant 3 3 0 3 Antisettling 1 1 1 1

The cement slurries were cured for 7 days at 53° C. and 20 MPa. Theself-healing test results are presented in FIG. 4. For both cementmatrices, density variation does not affect performance in terms offlow-rate reduction.

Example 5

Using the methods described in Example 3, the effect of pressure on theperformance of set cements containing SIS#1 or SBS#3 was investigated.The 1606-kg/m³ formulations from Table 9 were tested.

The samples were cured for 7 days at 53° C. and 20 MPa.Flow-rate-reduction measurements were performed at four methanepressures: 3.5 MPa, 7 MPa, 13.7 MPa and 20 MPa. The results, presentedin FIG. 5, indicate that flow-rate reduction was achieved at 3.5 MPa forthe set cement containing SIS, and at 7 MPa for the set cementcontaining SBS.

The invention claimed is:
 1. A cement slurry, comprising: thermoplasticblock-polymer particles selected from the group consisting ofstyrene-isoprene-styrene polymer particles, styrene-butadiene-styrenepolymer particles, and mixtures thereof; wherein the thermoplasticblock-polymer particles have a particle size between 100 μm and 900 μm,and a tensile strength between 1.5 MPa and 40 MPa, wherein the slurryfurther comprises an aqueous inverse emulsion of particles comprising abetaine group, poly-2, 2, 1-bicyclo heptene (polynorbornene),alkylstyrene, crosslinked substituted vinyl acrylate copolymers,diatomaceous earth, vinyl acetate rubber, polychloroprene rubber,acrylonitrile butadiene rubber, hydrogenated acrylonitrile butadienerubber, ethylene propylene diene monomer, ethylene propylene monomerrubber, styrene/propylene/diene monomer, brominatedpoly(isobutylene-co-4-methylstyrene), chlorosulphonated polyethylenes,brominated butyl rubber, chlorinated butyl rubber, chlorinatedpolyethylene, epichlorohydrin ethylene oxide copolymer, ethyleneacrylate rubber, ethylene propylene diene terpolymer rubber, sulphonatedpolyethylene or substituted styrene acrylate copolymers or combinationsthereof; the cement slurry to be pumped in a well with a boreholepenetrating a methane-containing formation, wherein methane is presentat a concentration higher than 91 mol % and at a pressure higher than3.5 MPa; and wherein the thermoplastic block-polymer particles swellwhen in contact with the methane, after microannuli, cracks or defectsoccur in a cement sheath.
 2. The cement slurry of claim 1, wherein theconcentration of the particles is between about 10% and 55% by volume ofcement-slurry solids.
 3. A cement slurry for maintaining zonal isolationin a subterranean well in which a borehole penetrates one or moremethane-containing formations, comprising: thermoplastic block-polymerparticles selected from the group consisting of styrene-isoprene-styrenepolymer particles, styrene-butadiene-styrene polymer particles, and amixtures thereof; wherein the slurry further comprises an aqueousinverse emulsion of particles comprising a betaine group, poly-2, 2,1-bicyclo heptene (polynorbornene), alkylstyrene, crosslinkedsubstituted vinyl acrylate copolymers, diatomaceous earth, vinyl acetaterubber, polychloroprene rubber, acrylonitrile butadiene rubber,hydrogenated acrylonitrile butadiene rubber, ethylene propylene dienemonomer, ethylene propylene monomer rubber, styrene/propylene/dienemonomer, brominated poly(isobutylene-co-4-methylstyrene),chlorosulphonated polyethylenes, brominated butyl rubber, chlorinatedbutyl rubber, chlorinated polyethylene, epichlorohydrin ethylene oxidecopolymer, ethylene acrylate rubber, ethylene propylene diene terpolymerrubber, sulphonated polyethylene or substituted styrene acrylatecopolymers or combinations thereof; wherein the particles have aparticle size between 100 μm and 900 μm, and a tensile strength between1.5 MPa and 40 MPa.
 4. The cement slurry of claim 3, wherein theconcentration of the particles is between about 10% and 55% by volume ofcement-slurry solids.
 5. The cement slurry of claim 3, wherein theparticles swell in response to contact with the methane.
 6. The cementslurry of claim 3, wherein the methane in the methane-containingformation is present at a concentration higher than 91 mol % and at apressure higher than 3.5 MPa.